Solid state relays provide a more reliable and faster means to switch loads on and off in a secondary region. When compared to mechanical relays, solid state relays have a much longer cycle life and can turn loads on and off much faster. One of the challenges with solid state relays is that they require a constant charge on the control gate. As such, this constraint requires the control circuit to be constantly operating, which can reduce the battery life when the control circuit is a portable device, as is often the case when a separate power source is not provided for the control circuit.
Many conventional solid state relays use a capacitor to isolate its solid state switches from the primary side to the secondary side. In 24-volt alternating current (VAC) systems, the ground line is not available, and only the VAC line (common) and the load lines are available. Because there is no ground line, there is no robust supply path on the secondary side. Therefore, the power to charge the gates of the switches is provided by continuously clocking the isolating capacitor that is tied to the solid-state relay. This continued clocking consumes continuous power from the primary side. In many cases, the primary side is battery powered, and therefore, the battery power is consumed.
Solid-state relays can be used for driving 24 VAC systems, including but not limited to heating, ventilation and air conditioning (HVAC) control systems, security systems, smoke detector systems, water sprinkler control systems and pool control systems. These systems can be configured to use isolated and non-isolated power systems. Because there is no supply and ground line located at the secondary side of the solid state relay, energy can be alternatively received by rectifying the energy across the common line and a load line using a diode bridge or other suitable circuits. When a switch is in the off position, the diode bridge can provide energy across the rectifier. A problem with this architecture is that if too much load current is drawn in the off-state switch, the load can inadvertently turn on, because the load current threshold for switching on the load is met. This is undesirable and can cause the system to work in an incorrect manner. Alternatively, energy transfer across a rectifier or other suitable power converter can be limited to occur only when the switch is in the off-state. Therefore, when the switch is in the on-state, energy will not be transferred across the rectifier.